Conventional liquid crystal display devices include various types of liquid crystal display devices such as an active matrix liquid crystal display device using thin film transistors (TFTs) as switching elements, and a simple matrix liquid crystal display device of STN and the like. Although these liquid crystal display devices differ in the ways they drive liquid crystal, they have in common the fact that liquid crystal is sealed between opposed glass substrates, that signal lines are disposed in a matrix form on the glass substrate, and that the liquid crystals in the vicinity of the intersecting signal lines are driven to construct display pixels.
In addition, the liquid crystal display panel of these conventional liquid crystal display devices has an image display area disposed so as to be exposed in the opening of the case covering the liquid crystal display panel, and a frame area which is the area between the edge portion of the case opening and the periphery or the image display area. The frame area is an area needed for preventing the chipping of the image display area due to the thickness of the case covering the liquid crystal display panel, for instance, when the liquid crystal display area is obliquely observed. The width of the liquid crystal display panel is about 2 to 3 mm, for instance, for a liquid crystal display device whose image display area is 10 inches in size. From a signal driver circuit provided outside the frame area, signal lines are disposed in a matrix form over the frame area into the image display area. Since the frame area is an area having no relationship to the image display and the display quality of the liquid crystal display device is degraded if light is transmitted from the frame area, it requires a blocking process.
The conventional blocking process for the frame area is described below.
The most typical method is to form a material having a blocking ability in the frame area on one or both of the opposed glass substrates of the liquid crystal display (LCD) panel. For instance, in most TFT/LCD panels, a blocking layer is formed by depositing a material having a blocking ability, for instance, a metal material such as Cr, in the frame area of an opposing substrate having a common electrode formed thereon and opposed to an array substrate having TFTs formed thereon.
However, in this method, the reflection by the blocking layer of the metal material of the opposing substrate is marked, which leads to the degradation of the display quality. In addition, it also has a problem in that the manufacturing process of the liquid crystal display device is made more cumbersome in accordance with the need for a blocking layer formation process only for blocking.
In contrast to the above described method which is general, but has problems, a method for blocking the frame area which requires no additional process for forming a blocking layer is disclosed, for instance, in the PUPA 61-32087 official gazette. The blocking methods disclosed in this official gazette and the like are as follows: (1) A blocking area is formed by extending the width of signal lines having a blocking ability passing through the frame area (for instance, metal wirings) to the vicinity of the adjacent signal lines in the frame area, (2) island-like blocking films are formed between the signal lines in the frame area by the same process as the wiring formation, and, (3) since the lateral wirings and the vertical wirings are formed in a multilayer wiring structure, blocking is performed by forming a blocking layer in which the frame area through which the lateral wirings are passing is paved with the vertical wiring formation material, forming a blocking layer in which the frame area through which the vertical wirings are passing is paved with the lateral wiring formation material. In these blocking methods, however, in methods (1) and (2), a gap must be left between the wirings and the area to be blocked to prevent short-circuiting between the wirings, there is a problem in that light leaks from this gap, and the blocking ability is incomplete. Further, in method (3), since the overlapping area of the signal lines and the paved blocking layer is large, the stray capacitance is large for the signal lines, which causes a problem in that a signal delay is caused that results in the degradation of the picture quality. In addition, since short-circuiting between the signal lines and the blocking layer is very likely to become a defect fatal to the display panel, the problem of a reduction in the manufacturing yield also occurs.
Further, in an article titled "Liquid Crystal Display" by T. Fukunaga et al., IBM Technical Disclosure Bulletin, Vol. 36, No. 5, May 1993, pp. 495-496, a method is disclosed in which a blocking layer is formed in a frame area using the wiring material for X-drive lines and Y-drive lines in a manner similar to method (2) described above, and the entire surface of the frame area is coated with a black organic material to increase the blocking ability. However, even this cannot prevent light leakage from the gap in the metal layers, which leads to a problem that higher picture quality cannot be obtained.
In addition, since the frame area is an extra area which has no relationship to the image display as described above, a protection circuit is formed using this area for preventing electrostatic failure which is a problem in the manufacture of the liquid crystal panel. Electrostatic failure, which the above-mentioned problem in the manufacture process of a liquid crystal panel and preventive measures against it will now be briefly described. Electrostatic failure is a problem particularly in the TFT/LCD (TFT liquid crystal display device). That is, since the gate wirings and the data wirings of a TFT array are electrically insulated by an insulation layer, in the manufacturing process of the display panel of a TFT/LCD, they are easily electrostatically charged for various reasons, such as friction from the wind blowing in a clean room, for instance, during a process in the clean room. When the gate lines or data lines are electrostatically charged, a voltage considerably larger than the actual driving voltage is applied to TFTs and the intersections of the wirings, causing dielectric breakdown of the insulation film, and as a result, they cannot function as switching elements or a fatal defect such as a line defect is caused.
To prevent such electrostatic failure, conventionally, for instance, a protection circuit is formed as disclosed in the PUPA 63-220289 official gazette. This is characterized in that reference potential wiring is provided and electrically connected to each signal wiring separately by a two-terminal operation switching element. This necessarily occupies a large area dedicated to the protection circuit so that the switching element has an appropriate resistance value as a protection circuit. In a circuit of this type, to prevent reduction in the manufacturing yield such as short-circuiting of the wirings, it is necessary to leave the gap of the metal layer in the vicinity of the protection circuit, conflicting with the blocking ability, and thus it cannot be formed in the frame area unless there is a blocking layer on the opposing substrate side. In consequence, this circuit must be formed outside the frame area, but it is usually difficult for various reasons to acquire such area outside the frame area.
In the protection circuit disclosed in the PUPA 62-65455 official gazette, adjacent wirings in the frame area are electrically short-circuited by a high-resistance connector. This high-resistance connector can be formed in the frame area. However, since the high-resistance connector constituting the circuit has substantially linear current-voltage characteristics, the display quality is reduced by the interaction between the signal lines if the resistance value is decreased, and the capability of preventing electrostatic failure becomes smaller. Consequently, it has a problem in that the display screen quality and the prevention of electrostatic failure become incompatible with each other.
As described above, conventional liquid crystal display devices have various problems to be solved in the blocking ability of the frame area, and the ability to prevent electrostatic failure in the protection circuit formed in the frame area.